In general, solid golf balls include a one-piece golf ball which is integrally molded in its entirety and a multi-layered golf ball having a core enclosed in a cover with or without one or more intermediate layers. The one-piece golf balls in their entirety and the multi-layered golf ball cores are often manufactured from a rubber composition comprising a rubber component such as polybutadiene, a co-crosslinking agent such as a metal salt of an unsaturated fatty acid, a weight modifier such as zinc oxide, and a radical initiator such as dicumyl peroxide by molding the composition under heat and pressure.
Most rubber compositions use zinc methacrylate or zinc acrylate as the unsaturated fatty acid metal salt for the co-crosslinking agent because higher ball hardness and better repulsion are obtained. The amount of zinc methacrylate or acrylate blended is usually as large as 15 to 60 parts by weight per 100 parts by weight of a rubber component such as polybutadiene. In preparing a rubber composition by milling the respective ingredients, which are usually available in fine powder form, several problems arise including scattering of powder ingredients, difficulty of milling operation due to adhesion to a mill, especially to rolls, and poor dispersion due to potential agglomeration in the composition. These working defects prevent full utilization of the zinc methacrylate or acrylate blended, resulting in balls having inconsistent hardness and repulsion, sometimes an extremely low hardness.
Therefore, an object of the present invention is to provide a solid golf ball having minimal variations in hardness and repulsion and exhibiting consistent flying performance.